Taeniid cestode infections in man and animals constitute a serious zoonotic and economic problem throughout the world. Fundamental to the continuity of the life cycles of these parasites is the ability of the larval stages to survive for extended periods of time within the tissues of mammalian intermediate hosts. That they are able to do so in immunologically competent animals, which show a marked and specific antibody-mediated immunity to superinfection, is a paradox challenging to our understanding of resistance mechanisms in infectious disease. The purpose of this project is to examine immunological events during taeniid metacestode infection. Particular emphasis will be placed on interactions between parasite membranes, antibody, complement, and both specific and nonspecific effector cells. Ultrastructural, pathological, and immunochemical techniques will be used to explore the mechanisms involved in immunologic attack on the parasite, and the means whereby established parasites are able to retain control of membrane function and preserve membrane integrity in the face of immunologic reactions on the part of the host. Growth and development in normal and immune hosts will be studied in an effort to determine the factors which characterize host cell reactivity in infected tissues. The immunogenicity of well defined parasite products will be studied. Studies will include the characterization of antigens which are able to immunize rats orally. The adjuvant effect of B. pertussis will be examined and the site at which immune reactions occur in orally or parenterally immunized rats will be examined. The effects of parasite products on coagulation and on gastro-intestinal cell turnover will be determined.